Solji, an elementary school student, likes Yerin in the same class. She bravely confessed, but Yerin refuses, and Solji becomes a tease. In the meantime, Solji starts a new math workbook and meets Jin-ah teacher. To Jin-ah, who is gently approaching her, Sol-ji opens her heart. However, when Yerin finds out that she is also doing the same workbook as herself, she feels jealous because she seems to be closer to Jin-ah than she is, and fights with Yerin.